This invention relates to a method and means for measuring the level of a liquid in a container. More particularly, a coaxial cavity having a perforated tubular outer conductor is partially submerged in the liquid in the container so that liquid enters and terminates a portion of the annular region of the coaxial cavity. The fundamental resonant frequency of the portion of the coaxial cavity which does not contain liquid is determined experimentally and is used to calculate the length of the liquid-free portion of the coaxial cavity, and thereby the level of the liquid in the container.
It is extremely important from a safety standpoint to have accurate measuring devices for monitoring the level of liquid radioactive waste in large storage containers. Due to the highly radioactive nature of the liquid being stored a reliable measuring device is needed to insure against leaks or theft of the tanks' potentially dangerous contents.
One type of measuring device typically used to measure the level of liquid in a container which is well known to those skilled in the art is floats suspended within the liquid in the container. These floats can be connected to the outside of the container by mechanical linkage.
Another type of measuring device typically used in chemical processing plants is pneumatic bubble probes. These bubble probes can be connected to a pair of differential pressure transducers which give the data necessary to derive the level of the fluid in the container. In most applications this is a reliable approach, difficulty being encountered only in the case of solution near saturation which can cause the pneumatic bubble probes to clog.
One particular processing plant which uses pneumatic bubble probes as described above is the Idaho Chemical Processing Plant which is part of the Idaho National Energy Laboratory owned by the Department of Energy and located 50 miles west of of Idaho Falls, Idaho. The processing plant has a tank farm which contains several underground liquid-waste-storage containers. The containers are used to store a highly radio-active acidic solution which is in an intermediate step of reprocessing. It has been determined that a second independent measuring system is needed for monitoring the level of liquid in the storage containers. Safety considerations, in addition to the possibility of the loss of tank level information due to probe clogging, make it desirable that the second system not use pneumatic bubble probes.